I've seen tonneau covers made with steel electrical conduit, both 1/2
and 3/4 inch diameters were used for the bows. Depending on how you want
it to look, you may have to bend the bows differently.
For instance, some of them have a sharp, pronounced peak in the center
which you could probably do with a conduit bender, which you can find at
most larger hardware stores. For a gentler bend in softer material, you
might be able to bend it over your knee, but this is hard to do
consistently.
I've also used a form block, made from plywood, bolted down to a firm
surface, either a table or sheet of 3/4 plywood. Put an end stop on one
side that is half of the overall part length away from the center of the
form block. Pull on the part at the other end of the part at a distance
half of the part length away from the center of the form block and take
care to note how far you have pulled it, so that you can repeat it if it
produces the piece you want.
Hope the diagram helps.

-
- pull back this end
- v
- -------------------------\
- o \
o o \
o form o\
o block o\
\
\ end stop
\hold this end

A few pieces of aluminum cut over-length and forced into pockets in the
sidewalls will do it. Or simpler; wood

Thanks for all the replies
I'll let you know how it turns out.

Jason